The present invention relates to a process for the production of electronic parts, in which electronic circuits are formed on both the surfaces of a semiconductor substrate, the electronic circuits being for attaining an intended function. More specifically, it relates to a process for the production of electronic parts, which process uses a heat-resistant thermoplastic for supporting a semiconductor substrate on one surface of which electronic circuits are formed, so that the process can be applied to an electronic part production process including the step of processing (polishing and forming circuits on) exposed surfaces of the semiconductor substrate at a high temperature of 350xc2x0 C. or higher.
In recent years, it is increasingly required to decrease electronic machines and equipment in thickness and weight, and these machines and equipment are decreasing in thickness further and further, as is typically found in cellular phones and IC cards.
As a thin printed wiring board, printed wiring boards using a wholly aromatic polyamide paper or a polyimide film as a substrate are increasing in number.
Further, ceramic substrates are also available, and they are required to have a thickness of 0.2 mm or less, or they are required to have a smaller thickness of 0.1 mm, 0.05 mm, 0.03 mm or the like. However, a ceramic is generally hard and not deformable, and it has a defect that it is liable to break when decreased in thickness, unless it is a flexible thin glass sheet. For this reason, a ceramic substrate according to a thin film method has a thickness of 0.2 mm and a size of 50 mmxc3x9750 mm as a largest work size.
Similarly, electronic parts per se are decreasing in size according to demands for a decrease in size and higher functions.
With regard to silicon wafers, developments for increasing the work size from 20.32 cm to 30.48 cm are actively under way from the viewpoint of an increase in productivity. However, it is difficult to handle silicon wafers when they are decreased in thickness. Further, there is found no process in which metal-containing electronic circuits are formed on both the surfaces of a substrate at the same time, and in a presently available production process, an electronic circuit is formed on one surface and then an electronic circuit is formed on the other surface. Since the thermal expansion coefficient of metals such as copper and aluminum greatly differs from that of a semiconductor substrate by 10 to 15xc3x9710xe2x88x926Kxe2x88x921, a substrate warps when the substrate is decreased in thickness, and the substrate may break in some cases.
In the production of an electronic part having a thin semiconductor substrate and electronic circuits formed on both the surfaces of the substrate, therefore, it is required to employ, for example, a method in which semiconductors and other electronic circuit portions whose formation requires high temperatures are formed on one surface (e.g., front surface) of a semiconductor substrate having a general thickness, the substrate is supported by a supporting substrate by attaching the surface (in which semiconductors and other electronic circuit portions are formed) to the supporting substrate, the other surface (reverse surface) is ground and polished to decrease the substrate in thickness, electronic circuits are formed on the reverse surface, the substrate is peeled from the supporting substrate, and the substrate is separated into individual electronic parts.
In the step of forming electronic parts on the reverse surface, processing at a high temperature is not required so long as metallizing is carried out to such an extent that a difference between the thermal expansion coefficients is balanced. However, when semiconductor circuits are formed, a substrate is required to have durability against a high temperature of approximately 350xc2x0 C. or higher and a vacuum degree at which plasma processing or ion plating can be carried out at such a temperature.
Then, it is required to employ a method in which the thin and fragile semiconductor substrate in which a number of electronic circuit parts are formed is peeled off from the supporting substrate with taking care not to break the substrate, and the substrate is separated into individual electronic parts.
It is therefore required to provide a supporting substrate which can be used in the above steps and can be used repeatedly to some extent and which has a thermal expansion coefficient substantially equivalent to the thermal expansion coefficient of a semiconductor substrate, an adhesive which serves to support the semiconductor substrate on the supporting substrate and which permits peeling the semiconductor substrate off from the supporting substrate, and a method of use of these.
It is an object of the present invention to provide a method which endures the step of grinding and polishing and the step of processing semiconductor circuits at a temperature of 350xc2x0 C. or higher and can separate electronic parts. In this object, it is to provide a supporting substrate which endures a repeated use, a method of supporting by bonding and a separation method of, electronic parts.
According to the present invention, there is provided a process for the production of electronic parts, comprising the steps of
forming semiconductor circuits in one surface (surface A) of a semiconductor substrate (SEC) having a thickness of at least 0.2 mm,
supporting the semiconductor substrate on a supporting substrate (BP) by bonding (AS) of said surface A to the supporting substrate (BP),
grinding and polishing the exposed other surface (surface B) of the semiconductor substrate (SEC) by a physical method, a chemical method or a method of combination of these methods, to decrease the thickness of the semiconductor substrate (SEC) to less than 0.2 mm,
forming semiconductor circuits in the polished surface, to obtain an electronic-circuits-possessing semiconductor substrate (PSE), and
peeling (PS) the electronic-circuits-possessing semiconductor substrate (PSE) off from the supporting substrate (BP),
wherein the step of polishing the surface B or the step of forming electronic parts in the surface B includes the step of processing the surface B at a high temperature of at least 350xc2x0 C., and the bonding (AS) uses a heat-resistant thermoplastic (RF).
In the present invention, preferably, a difference between the thermal expansion coefficient of the semiconductor substrate (SEC) and the thermal expansion coefficient the supporting substrate (BP) is 2xc3x9710xe2x88x926Kxe2x88x921 or less, the heat-resistant thermoplastic (RF) is selected from the group consisting of polyimide, polyetherimide, polyamideimide, polyesterimide, polyether ether ketone, polyester and polyamide, the bonding (AS) is carried out by thermal pressing under conditions of a temperature of 150 to 400xc2x0 C., a pressure of 0.1 to 5 MPa and a time period of 3 to 90 minutes, the peeling (PS) is carried out after treatment with water, an amine or a mixture of water with an amine, an ultrasonic treatment at 28 kHz to 150 kHz is also carried out in combination for the peeling (BS), and the supporting substrate (BP) is a resin composite inorganic substrate prepared by impregnating an inorganic substrate selected from the group consisting of an aluminum nitride-boron nitride (AlN-h-BN) substrate, an aluminum nitride-silicon carbide-boron nitride (AlNxe2x80x94SiCxe2x80x94hxe2x80x94BN) substrate, an alumina-boron nitride (Al2O3xe2x80x94hxe2x80x94BN) substrate, a substrate made of a xcex2-silicon carbide porous material, an amorphous carbon substrate and a carbon-fiber-reinforced carbon substrate with a heat-resistant resin and curing the resin.
The present invention will be explained in detail hereinafter.
Semiconductor Substrate (SEC) Having a Thickness of at least 0.2 mm
The semiconductor substrate (SEC) for use in the present invention is typified by a silicon (Si) wafer. In addition, the semiconductor substrate (SEC) includes a semiconductor substrate containing an element such as germanium (Ge), selenium (Se), tin (Sn) or tellurium (Te), and a semiconductor substrate containing gallium-arsenic (GaAs) or containing other compound such as GaP, GaSb, AlP, AlAs, AlSb, InP, InAs, InSb, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, AlGaAs, GaInAs, AlInAs or AlGaInAs.
Supporting Substrate (BP)
The supporting substrate (BP) is required to have durability against a high temperature of at least 350xc2x0 C., and it is further essentially required to have durability against the step of grinding and polishing such as a lapping step and chemicals used for the pre-treatment and post-treatment for forming electronic circuits. Further, when the supporting substrate (BP) is used, the supporting substrate (BP) is essentially required to have a thermal expansion coefficient substantially equivalent to a thermal expansion coefficient of the semiconductor substrate.
Specifically, the supporting substrate (BP) is selected from an alumina substrate, an aluminum nitride substrate, a silicon carbide substrate, a silicon nitride substrate, a borosilicate glass substrate or various carbon substrates.
In the present invention, the supporting substrate (BP) is preferably a substrate prepared by selecting an inorganic continuously porous substrate having a continuous pore content of at least 0.5% by volume, more preferably 2 to 35% by volume and having an average pore diameter of 0.1 to 10 xcexcm, impregnating the inorganic continuous porous substrate with a heat-resistant resin and curing the heat-resistant resin.
Specifically, the inorganic continuously porous substrate is made of an inorganic material selected from an aluminum nitride-boron nitride (AlN-h-BN), an aluminum nitride-silicon carbide-boron nitride (AlNxe2x80x94SiCxe2x80x94hxe2x80x94BN), zirconium oxide-aluminum nitride-boron nitride (ZrO2-AlN-h-BN), alumina-boron nitride (Al2O3-h-BN), zirconium oxide-alumina-boron nitride (ZrO2-Al2O3-h-BN), silicon nitride-boron nitride (Si3N4-h-BN), alumina-titanium oxide-boron nitride (Al2O3-TiO2-h-BN), a xcex2-silicon carbide porous material (xcex2SiC), amorphous carbon or carbon-fiber-reinforced carbon.
The heat-resistant resin used for impregnating the inorganic substrate can be selected from those addition-polymerizable or crosslinking heat-resistant aromatic polyfunctional cyanate ester resins disclosed in JP-A-8-244163 and JP-A-9-314732 by the present inventors and some others. Of these, highly heat-resistant silicone resin is particularly preferred as a heat-resistant resin feasible for use at a high temperature of over 350xc2x0 C.
Before the inorganic substrate is impregnated with the above resin, it is preferred to treat the surface of the inorganic substrate for improving the affinity between the inorganic substrate surface including inner wall surfaces of the continuous pores and the resin. The surface treatment is preferably carried out as follows. A solution of an organometallic compound containing aluminum, titanium or silicon or an organometallic compound which is a prepolymer having a weight average molecular weight less than 10,000, generally, in an organic solvent is prepared, the inorganic substrate is impregnated with the organic solvent solution under vacuum, the impregnated substrate is air-dried to remove the solvent and pre-heated, and the organometallic compound is pyrolyzed at a maximum temperature of 850xc2x0 C. or lower. (See U.S. Pat. No. 5,686,172).
When the above heat treatment is carried out, the substrate is improved in affinity thereof to the resin to be used for the impregnation, and further, the substrate is remarkably improved in adhesion thereof to the heat-resistant thermoplastic (RF) used for the bonding.
In the present invention, the supporting substrate (BP) can be repeatedly used. That is, the semiconductor substrate (SEC) having circuits formed on both the surfaces thereof is separated from the supporting substrate (BP), the adhesive resin is separated from the supporting substrate (BP), and the supporting substrate (BP) is cleaned, re-impregnated, re-ground and re-polished as required and can be used as a supporting substrate again, which is essential in view of effective use and economic performance. Naturally, the above supporting substrate for a re-use may be also applied to a process including no step of processing at a high temperature of at least 350xc2x0 C.
Heat-resistant Thermoplastic (RF)
A semiconductor substrate (SEC) having electronic circuits formed on one surface thereof is supported on the above supporting substrate (BP) by bonding (AS) of the electronic-circuits-possessing surface of the semiconductor substrate to the supporting substrate (BP), the semiconductor substrate (SEC) is processed in a step of predetermined processing procedures including high-temperature processing at 350xc2x0 C. or higher, and the semiconductor substrate (SEC) is peeled off, whereby semiconductor parts are produced.
The bonding (AS) is required to be carried out by a method which satisfies the following essential requirements; the supporting substrate (BP) is not to be separated in the step of the predetermined processing procedures, and the semiconductor substrate (SEC) can be easily peeled off after the step of the predetermined processing procedures. Further, it is essentially required to satisfy that the semiconductor substrate (SEC) and the supporting substrate (BP) attached to each other by the bonding (AS) are not to be warped during the step of the predetermined processing procedures due to a difference in thermal expansion coefficient.
In view of the above points, for the bonding (AS) in the present invention, it is preferred to select a heat-resistant thermoplastic (RF). Specifically, the heat-resistant thermoplastic (RF) can be selected from polyimide, polyetherimide, polyamideimide, polyether ketone, polyether ether ketone, polyesterimide, polyester, polyamide, a liquid crystal polymer, polyether sulfone, polysulfone or polyphenylene sulfide.
The method of utilizing these resins for the bonding includes (1) a method using a film having a thickness of 10 to 100 xcexcm or (2) a method comprising applying a resin solution by a film-forming method such as spin coating and drying the applied resin solution to form a film having a thickness of 20 xcexcm or less. Preferably, these methods are properly used alone or in combination depending upon a purpose.
The step of using water for the peeling is a useful method. In this case, it is preferred to use a heat-resistant thermoplastic selected from the above heat-resistant thermoplastic having a water-absorption rate of at least 1%.
In view of absorption of a concave and convex shape of electronic parts in which circuits are formed, the thickness of the adhesive resin (RF) is at least 10 xcexcm, more preferably at least 15 xcexcm. The maximum thickness of the resin (RF) generally does not exceed 100 xcexcm since the maximum thickness that can be applied to the processing step is limited in view of the thickness of the supporting substrate.
Processing Step
Electronic circuits are formed on one surface (surface A) of the semiconductor substrate (SEC), and electronic circuits are formed on the other surface (surface B). Generally, the processing step essentially includes the step of processing with a machine, the step of cleaning with an acid and the step of heating at 350xc2x0 C. or higher.
Step of processing with a machine:
Generally, the semiconductor substrate (SEC) is decreased in thickness and flattened. For this reason, the heat-resistant thermoplastic (RF) is required to have durability against polishing solution for lapping and chemical mechanical polishing (CMP) and mechanical vibration. Further, the heat-resistant thermoplastic (RF) is required to protect the semiconductor circuits from the polishing solution and the mechanical vibration which are accompanied with these steps.
Step of cleaning with an acid:
The heat-resistant thermoplastic adhesive film is required to have durability against a cleaning step using an inorganic acid such as hydrochloric acid, phosphoric acid, nitric acid, sulfuric acid or the like.
Step of heating at 350xc2x0 C. or higher:
Circuits made of metal thin films are formed, so that the heat-resistant thermoplastic adhesive film is required to have durability against the steps of chemical vapor deposition (CVD) and ion plating. In this case, no force is externally applied. However, the heat-resistant thermoplastic adhesive film is required to be feasible for use at a high temperature under vacuum, and it is required to be almost free from generating outgas.
Naturally, the step of bonding (AS) by pressing under heat approximately at 300 to 400xc2x0 C. under pressure and the step of separating, generally, with a hot aqueous solution will follow.
Bonding (AS)
In the present invention, the bonding (AS) is required to be effective during at least the above three steps.
For the above reason, in the present invention, the heat-resistant thermoplastic film is used, and the semiconductor substrate (SEC) is generally bonded to, and supported on, the supporting substrate (BP) by hot pressing under reduced pressure at 300 to 400xc2x0 C. under a pressure of 1 to 80 kg/cm2 (0.1 to 8 MPa), preferably 1 to 50 kg/cm2 (0.1 to 5 MPa), more preferably 5 to 30 kg/cm2 (0.5 to 3 MPa).
There are some heat-resistant thermoplastic films that can be used for bonding at a temperature lower than 300xc2x0 C. but have no durability against the heating step at a temperature of 350xc2x0 C. or higher. While the bonding at a higher temperature results in better results, a high temperature exceeding 400xc2x0 C. is generally not required.
Method of Separation (peeling)
The semiconductor substrate (SEC) having circuits formed and the supporting substrate (BP) are separated from each other by a method in which the semiconductor substrate (SEC) and the supporting substrate (BP) which are attached to each other by bonding (AS) are immersed in water, an amine or a mixture solution of water and an amine. When ultrasonic treatment is used in combination, the time period for the separation is decreased. Heating (25xc2x0 C. to 140xc2x0 C.) is preferred for improving absorption of water. While the water may be water generally available, it is preferred to use pure water in view of infiltration and prevention of contamination of the substrate.
The amine can be selected from various amines such as aliphatic amines, aromatic amines and heterocyclic amines, and these amines are preferably soluble in water and hot water. The aliphatic amines include methylamine, tert-butylamine, sec-butylamine, n-butylamine, n-propylamine, isopropylamine, dimethylamine, diethylamine, triethylamine, diethanolamine and 2,5-dimethyl-2,5-hexamethylenediamine. The aromatic amines include aniline, diphenylamine, xylenediamine, dimethylaniline and p-toluidine. Further, other compounds such as ammonia, 1,6-dicyanohexane and morpholine may be used.
A hydrazine/KOH solution, which is often used for etching a polyimide film, shows a very high peeling rate. Since, however, it corrodes silicone, etc., generally, it cannot be used. However, it may be used as a remarkably excellent peeling medium depending upon some semiconductor substrate and some surface treatment thereof.
For the ultrasonic treatment, 28 to 150 kHz may be used. Generally, 50 to 100 kHz is easy to use. The ultrasonic treatment is preferably carried out under heat at 40 to 80xc2x0 C. When the ultrasonic treatment is carried out for a long period of time, it is required to discontinue the treatment pulsatively so as to prevent an excess increase in temperature.
Concerning the order of separation of the supporting substrate (BP) and the semiconductor substrate (SEC) having circuits formed, the semiconductor substrate (SEC) is first separated from the heat-resistant thermoplastic (RF) when the semiconductor substrate has been decreased in thickness. When the adhesive resin remains on the semiconductor substrate, the substrate is warped due to a residual stress of the resin. When the semiconductor substrate is warped to a great extent, the electronic-circuit-possessing semiconductor substrate decreased in thickness undergoes cracking and is eventually destroyed.
The electronic-circuits-possessing semiconductor substrate (PSE) is generally cut into chip sizes by a method using a dicing saw having a diamond blade having a blade thickness of 100 xcexcm or less, while other methods such as a laser, etc., may be used. Further, in the cutting of the electronic-circuits-possessing semiconductor substrate (PSE) into individual chip sizes, a preparatory cut such as a V-cut or a U-cut is properly carried out for preventing the occurrence of chippage due to a cutting failure.
Concerning the above-mentioned peeling, there is a method comprising bonding a tape, such as a tape for peeling or for dicing, to the surface B of the electronic-circuits-possessing semiconductor substrate (PSE), fixing a tool, such as a stainless sheet for dicing, on the tape and peeling the electronic-circuits-possessing semiconductor substrate (PSE) off in this state. The above method is free from a warpage and decreases the occurrence of a breakage. So, the peeled electronic-circuits-possessing semiconductor substrate (PSE) can be used for cutting as it is.
Further, it is one of preferred methods to carry out the peeling after the electronic-circuits-possessing semiconductor substrate (PSE) is cut into individual chip sizes.
In this case, the electronic-circuits-possessing semiconductor substrate (PSE) bonded to the supporting substrate is fixed on a table such as a dicing saw, cut into individual chips and then peeled off. Here, reference point(s) for a cutting position are formed on the electronic-circuits-possessing semiconductor substrate (PSE) in advance, and the reference point(s) are optically read. It is preferred to form reference point(s) on the pattern of the surface A which is a protective surface or on the surface A and to read the reference point(s) optically. There may be used a transmission wavelength band of the semiconductor substrate, e.g., a wavelength of 1.3 xcexcm in the case of a gallium-arsenic substrate or a wavelength of 1 xcexcm in the case of a silicon substrate.
As described above, the present invention uses the heat-resistant thermoplastic for bonding and supporting of the semiconductor substrate having electronic circuits on one surface to/with the supporting substrate, and the present invention can be therefore applied to a method of producing electronic parts which method comprises the step of processing at a high temperature of 350xc2x0 C. or higher for polishing an exposed surface of the semiconductor substrate and forming electronic circuits on/in the surface.
The step of decreasing the semiconductor substrate in thickness in the present invention can be applied to the preparation of a thin ceramic sheet.